Rotating machinery often includes components such as hubs of impellers, disks, or rings that are thermally shrunk on a circular shaft. When these shrunk-on components are subjected to severe cylic loading paths, such as can be induced by large changes in rotational speed and/or large thermal transients such as can occur with the start-up or shut down of high temperature, high speed machinery, such as with large induced draft hot gas fans used to handle combustion gases in a power plant, or gas or steam turbines, these shrunk-on components occasionally "ratchet" or "walk" up or down the shaft. The main reason for this is because of the differential expansion between the component and the shaft, due to large thermal transients for the most part. Thus, taking a centrifugal fan which is to handle hot combustion gases for example, as the fan is brought into service and begins at ambient temperature, the temperature of the fan wheel and hub increases faster than the temperature of the shaft upon which the hub is mounted. Then, when the fan is taken out of service, the wheel and hub temperature will decrease faster than that of the shaft. To the extent ratcheting occurs under these conditions, problems such as material fretting, unbalance, vibrations, and improper alignments arise.
Ratcheting has sometimes been viewed as an indication that the shrunk-on component had become loose (or nearly loose) sometime during a thermal transient. However, it has been concluded from analysis that ratcheting is caused by relative movement or slipping at the interface of the component and shaft, and that this ratcheting can occur even when the component is relatively tightly shrunk on throughout the thermal transient. In this connection it is noted that the degree of shrink fit is typically mathematically calculated to provide a sufficient radial force that the frictional force resisting expansion of the component relative to the shaft will at least equal or exceed the force arising from the coefficient of expansion with the contemplated elevated temperature of the component relative to the shaft.
It is the aim of this invention to provide arrangements for at least reducing the degree of ratcheting experienced in devices of the character noted before.